For some ducks, there really is more than meets the eye. In some species, their penises get longer when fighting with other males in order to mate with a female, according to a study.
The researchers, from Mount Holyoke College and other institutions, wanted to take a closer look at the penises of ducks that have different mating styles, Newsweek reports. So the team went to a nearby waterfowl conservancy and experimented by re-organizing the ducks.
There were two species involved: lesser scaups and ruddy ducks. Lesser scaups select their mates early in the season, and are often “nonaggressive and sociable.”
Ruddy ducks, on the other hand, mate for only a few days. The males fight with each other constantly throughout the mating season and are not known for their gentleness in coupling. However, they do have pretty baby blue bills, and possess long penises that have little hooks containing keratin.
For each species, the researchers arranged the ducks in two specific situations. In one, male and female ducks lived in pairs. In the other, individual females were put with two or three males. The team then measured penis lengths over two breeding seasons.
Even while peaceful, the ducks’ penises grew and shrank when mating season began and ended. But in the lesser scaups, males rooming with males grew longer penises than those living with a female.
Among the ruddy ducks, many of the males did not actually begin growing penises until their second year. When they did, they appeared to take turns, with each male duck reproductively ready in various windows during the breeding season.
National Geographic states that this could be because the ducks are subordinate, or non-alpha males.
The researchers did not delve into why the penis sizes change, and it’s left unclear as to whether either longer or shorter organs help a duck’s chances of mating successfully.
The study was published in The Auk.